need help adjusting workflow to git (or vice versa)

[Stephen Berman]

I'm learning how to use git and am still stumbling over basic things.
With bzr I had a shared repository of Emacs sources, containing several
branches, including a mirror of trunk that I only updated, and a branch
that I did my regular builds from, which I pulled from the trunk mirror
but occasionally made small changes to, which went into my builds.  I
also had a task branch for longer term development, and occasionally
copied or merged changes from there to the build branch.  All of these
branches had there own source files, which weren't touched by changes in
the other branches (unless I merged or copied over changes).  Now I've
cloned the new git repository from Savannah and followed the
instructions on the Emacs Wiki to make a task branch.  But there is
still only one set of sources.  I do see the task branch and can check
it out, but when I added my changes from the corresponding bzr task
branch and then checked out master again, it shows the (still
uncommitted) changes.  I also made a build branch but when I check it
out, it likewise shows the uncommitted changes from the task branch, so
if I now do make, the changes from my task branch will AFAICT be
included in the build, which I don't want.  So how can I (i) keep a
pristine copy of trunk (master) and (ii) build with possibly some local
changes but without all changes from the task branch?

Steve Berman